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TPS26600-02EVM: Not shutting down in short circuit conditions

Tom Riley
Prodigy 10 points
Part Number: TPS26600-02EVM

Hi, 

I have a 48V 3phase motor controller in production that is having thermal issues (Bus Capacitors Poping). 

My team has isolated the root cause of this failure to 2 circuits, both of these circuits are 48V, protected with individual TPS26600 chips and should have an over-current limit of 1.1A.

The chips have the following setup: 

  • Vin = 48V Lion Battery (38V-54.6V)
  • CIn = 1uF (100V)
  • In1, In2, UVLO connected 
  • UVLO to OVP 1.37MOhm
  • OVP to RTN = 20KOhm (55v)
  • DVDT to RTN = 47nF 
  • Mode not Connected 
  • ILIM to Return = 11K (around 1.1A)
  • IMON = Not Connected 
  • SHDN = Not Connected
  • FLT = Not connected 
  • Out 1 & 2 connected together.

Please note there is no Cout, Is this an Issue? 

Are there any issues with this circuit layout? 

Kind Regards

Tom

  • 0
    TI__Guru 68555 points

    Hi Tom,

    Welcome to e2e and thanks for reaching out.

    1. Please share schematic and layout to understand better.

    2. What test are you doing that is causing issue?

    3. You are doing test on EVM?

    4. What is getting damaged?

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 10 points

    Hi, 

    Thanks for getting back so quickly. 
    1. Schematic is attached to this email
    2. Short Circuit Test. Vout shorted directly to GND on either of the chips results in the current draw above the rated ILIM of 1.1A 
    3. Not doing a test on EVM, We have production units in the real world failing due to short circuits. 
    4. The OP current is not being limited and this is causing damage to downstream components (Fly wires and Capacitors) 
    Kind regards
    Tom
  • 0 in reply to Tom Riley
    TI__Guru 68555 points

    Hi Tom,

    Thanks for your reply. I will get back on this before tomorrow EOD.

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    TI__Guru 68555 points

    Hi Tom,

    1. OVP threshold is not 55V based on current resistors of 1.37M and 20K, it is around 83.4V. Please check again.

    2. During short circuit due to transients on input and output side exceeding device abs max rating device can damage.  Please refer datasheet section 11.1 Transient Protection and follow the options for transient protection. It could be happening that device is failing in short due to abs max violation and high current is flowing through the short. It will be good to add some output cap also.

    3. Can you share zoomed in and zoomed out waveform like below?

    4. Is the issue replicating on multiple boards?

    5. Is tps26600 device fine after short circuit?

    Regards

    Kunal Goel

  • 0 in reply to Kunal Goel
    Prodigy 10 points

    Hi Kunal Goel,

    1. Thank you!! You highlighted a key design flaw, the OVP is set incorrectly.

    My issue is these boards are already potted so I can't access or re-work.

    Knowing these boards have OVP set incorrectly, Can this directly cause issues? - What is the failure mode?

    // V-in is a 48V Battery with a 'Max Voltage of 54.6V

    2. I have promising results with a prototype wiring harness including Schottky and Cap on the Chip outputs, Thank you!

    3. I'm unable to provide a scope trace as the units are potted and I can only access Vout, Would this be helpful?

    4. No theTPS26600 Device that fails is cooked after the event

  • 0 in reply to Tom Riley
    TI__Guru 68555 points

    Hi Tom,

    It is clear that you need TVS and Schottky in your application. During short circuit due to transients VIN or maybe VOUT abs max spec is violated. Please take care of this in future design.

    You have set OVP to 83.4V greater than recommended max of 60V of this device...so if somehow say 58V comes at input efuse output will also see 58V.Device will not fail in this case but your downstream load may fail from overvoltage.

    Device can fail in VIN to VOUT short mode when voltage on input pin crosses 60V. 

    Regards

    Kunal Goel